1. Field of the Invention
This invention relates to a sifter for sifting powder, e.g., a food article, a chemical, or a drug in a powder form.
2. Brief Description of Related Arts
In conventional chute sifters, powder commonly falls through a chute into a sieving chamber and is stirred by rotation of rotating blades attached to and arranged coaxially with a rotating shaft, which is located at the center of the sieving chamber and is rotated by means of a motor. Such conventional chute sifters are described, e.g., in Japanese Laid-Open Patent Nos. S63-69577, H03-131372, and H11-244784. A structurally similar pneumatic conveying in-line sifter with a rotating shaft and rotating blades is also known from Japanese Patent Publication No. 3492676. This sifter is effectively used for separation of a powdery substance from air in an air-powder mixture, classification of the separated powdery substance, and removal of foreign substances from the separated powdery substance.
However, in these conventional sifters, the rotating shaft located at the center of the sieving chamber has a fixed diameter that is smaller than the diameter of a sieve provided in the sieving chamber. The sieving chamber has a relatively wide space to enable a large flow of the powder or the air-powder mixture. Particularly as shown in FIG. 19, an excess load is applied to a partial area of a screen 170 in a sieve 107 corresponding to an angular range N from a 5 o'clock angle to an 8 o'clock angle. Namely, only the partial area of the screen 170 is effectively being used for sieving. The sieving chamber has too large of a space to sufficiently scoop up the powder by means of the rotating blades. The remaining area of the screen 170 other than the partial area corresponding to the angular range N is not effectively used for sieving. The powder is localized in the partial area of the angular range N. This undesirably accelerates deterioration of the screen and shortens the lifetime of the sieve, while limiting the sieving efficiency.
Conventional sifters also disadvantageously cause separation of powders in a powder mixture comprising various grain sizes, thus lowering the quality of the powder mixture. Conventional sifters also have problems of a large pressure loss and a relatively large amount of air used for sieving.